The Government has given in to demands from protesters in Islington that surface trains along the most densely populated part of the line would not be environmentally acceptable. When the final route is announced in the next fortnight, it will include an extra 2.1 miles of tunnel under the borough.
Forty per cent of the people affected by the line live in Islington and the Government has been won over by the argument that the tunnel would avoid large compensation claims from them.
However, pleas for extra tunnels by residents alongside other parts of the line through east London and Kent have been turned down because they all involve substantial extra cost. No other tunnel plans will be included in the final route. The most expensive of the options, a pounds 60m tunnel underneath Ashford has been ruled out, according to Sir Keith Speed, the local MP.
The Islington residents' victory means that there will now be a tunnel for the 15 kilometres (9 1/2 miles) from Barking in east London to just north east of the line's terminus at St Pancras.
There the line will separate into two to maximise capacity in and out of the station, which means that about 200 Islington homes will still be disrupted by construction and running of the line.
The Department of Transport has been convinced that original plans to run the link next to existing track used by British Rail's North London line would be as expensive as building a tunnel underneath it.
The Government will also publish its views on what stations should be built on the line: Ebbsfleet, betweeen Dartford and Gravesend is the most likely option as an international station and possibly Stratford for the commuter trains that will share the line. The terminus will be confirmed as St Pancras, where about pounds 1bn will have to be spent on adaptions for international trains.
Ministers, who are anxious to push on with the project now that the Channel tunnel is due to open within weeks, will also ensure that the published line of the route is safeguarded and cannot be altered.
The Department of Transport will then begin the search for a private sector partner. The company chosen will not be able to change the route.
Building should begin in 1997 and completion is due in 2002. The Governnment is expected to pay a maximum of pounds 1bn towards the cost of the line, a third of the total.